Ruby on Rails is just another skill. It makes you more valuable because employers know that you have the ability to learn other techniques for web development, not just PHP. This holds true for any other skill.
You don't really need to find someone that knows Rails, you just need someone that can follow your requirements to build the web site that you need in the time that you need it. This can be done by any decent PHP or Rails programmer. However, I give PHP the edge in that there are currently more PHP programmers and PHP jobs out there, so it may be easier to find a PHP programmer.